Travel-time methods, for example, the pulse radar method and the frequency modulation continuous wave radar method (FMCW radar), utilize the physical law by which travel distance equals the product of travel time and propagation velocity. In the case of fill level measurement, the travel distance corresponds to twice the separation between the antenna and the surface of the fill substance. The received signal, thus the transmission signal reflected on the surface of the fill substance, and its travel time are determined in the case of pulse radar based on the so-called echo function, respectively the digitized envelope curve. The envelope curve represents the amplitudes of the echo signals as a function of the distance “antenna—surface of the fill substance”. The fill level is calculated from the difference between the known distance between the antenna and the floor of the container and the distance between the surface of the fill substance and the antenna, as determined by the measurement. The transmitted, respectively received, signals are microwaves, which have a wavelength between 3 and 300 GHz.
German Patent, DE 10 2005 057 053 A1 describes a radar module for out-coupling a reference signal for a fill-level measuring device. The radar module includes a first transmission path for transferring an electromagnetic transmission signal from a source to an antenna. A tap couples a reference signal out from the transmission path. After the tap, the transmission signal passes through a delay and then a circulator, which forwards the transmission signal to the antenna. From the antenna, the transmission signal is then transmitted toward the fill substance, where it is reflected back to the antenna as received signal and fed to the circulator. There, the received signal is passed to a directional coupler, where it is added to the reference pulse.
Such a radar module is disadvantageous, when the fill level is near the antenna. When the transmission signal escapes from the circulator, it undergoes a DK value change. This leads in the vicinity of the antenna to disturbances, which superimpose on the received signal. The received signal cannot be correctly evaluated in this case, so that the measuring of the fill level is corrupted.